This invention relates to the manufacture of products, such as plastic bags, from elongate webs of thin, flexible, sheet material, in which the typical production line includes a plurality of sequentially located machines which feed the web continuously and perform a number of sequential manufacturing operations on the web. Not infrequently, manufacturing operations are required in which the web must be stopped, for example, with certain types of hole punches, hand cutters, sealers, stamp-type printers, grommeting devices or novelty attaching devices. When such stopped-web operations are required, it is essential that the web be stopped in a precise and correct position with respect to the manufacturing devices for performing a particular operation so that the operation will take place at the proper location on the web. It also is important for the device to be capable of operation at a speed equal to that at which the other machines in the production line cause the web to be advanced. It is among the general objects of the invention to provide an improved web interrupting machine of the type described.